1. Field of the Invention
The present invention relates to the field of a objective determination of the audible threshold, i.e., independent of the cooperation by the patient, by using Acoustically Evoked Potentials (AEP).
2. Description of the Related Art
The objective hearing test is used in particular with infants and small children since at that age the otherwise customary subjective audiometry which requires an active participation of the patient, is not yet possible.
It is known to automatically determine the hearing response with click stimuli, as disclosed in the patent DE 195 48 982.9 A1. A click stimulus only gives as a result the sum of the Early Acoustically Evoked Potentials (FAEP) in the frequency range of approximately 1000 Hz-8000 Hz. Hence, it is not possible to determine the audible threshold for specific frequencies.
Also known is the frequency-specific determination of the audible threshold based on sound pulse-evoked FAEP with additional notched-noise-masking (Stxc3xcrzebecher E, Wagner H, Cebulla M, Heine S, Jerzynski P. Rationelle objektive Hxc3x6rschwellenbestimmung mittels Tonpuls-BERA mit Notched-Noise-Maskierung {Rational objective determination of the audible threshold using sound pulse BERA with Notched-Noise masking}. Audiologische Akustik 1993; 32:164-176). With this method, only the measured data have so far been objective, whereas the registration (decision: response present/not present) has to be evaluated by an examiner. The problem of an objective evaluation is therefore not solved. An automatic control of the entire examination process is therefore not possible. Additional disadvantages are a long duration of the examination, possibly severe audible stress caused by the masking noise.
A method for determining the audible threshold using the amplitude modulation following response (AMFR) is known from Cohen LT, Richards FM, Clark GM. A comparison of steady-state evoked potentials to modulated tones in awake and sleeping humans. J. Acoust. Soc. Am. 1991; 90: 2467-2479, and Griffiths SK, Chambers RD. The amplitude modulation-following response as a audiometric tool. Ear Hear 1991; 12: 235-241.
AMFR is a so-called xe2x80x9csteady-state response.xe2x80x9d AMFR results are obtained without applying short acoustic stimuli (sound pulses, clicks) in short sequence, as is otherwise customary. Instead, the stimulus is represented by a continuous amplitude-modulated tone. The frequency of the modulated tone is referred to as carrier frequency (FTr), the frequency of the modulation signal as modulation frequency (FMod). FIG. 1.1 shows the 1000 Hz stimulus signal in the time domain, FIG. 1.2 in the spectral domain. The hearing test is performed in the range FTrxc2x1FMod. The response is represented by a continuous sinusoidal signal with a frequency that corresponds to the modulation frequency.
The AMFR is evaluated in the spectral domain.
A hearing test using AMFR has the following advantages as compared to the usual hearing test using FAEP:
The response to the stimulus is frequency-specific without additional masking.
While the customarily used FAEP is characterized by a broad and very variablespectrum, AMFR is limited to a single spectral line. An objective statistic measurement of the AMFR is therefore much simpler than with the other AEPs.
Typically, an electrical stimulus artifact is captured together with the acoustic stimulation via headphones during an EEG derivation, which when using customary AEP cannot be safely segregated from the physiological response either in the time domain or in the frequency domain. With AMFR derivation, the carrier and the sidebands separated by the modulation frequency produce an electrical artifact. Since the response to a single spectral line that is not in the frequency range of the carrier is limited, the response and the stimulus artifact can be separated reliably without any problem.
A simultaneous test with several frequencies is possible (Lins OG, Picton TW. Auditory steady-state responses to multiple simultaneous stimuli. Electroenceph. Clin. Neurophysiol. 1995; 96: 420-432), thus making the examination shorter than with the notched-noise BERA.
The known solutions have the following disadvantages:
The amplitude of the response to the stimuli is in the nanovolt range, in other words, very small. The AMFR has always a superimposed noise (EEG). The signal-to-noise ratio (SNR) is also very small. This applies particularly in situations close to the audible threshold since the amplitude of the response decreases with decreasing stimulus level. In addition, the response amplitude has a relatively large scatter between individual patients. This causes a corresponding scatter for determining the response near the threshold. This scatter is too large for practical applications. To increase the response amplitude, Cohen et al. (1991) also amplitude-modulated a frequency-modulated carrier. However, the amplitude gain as compared to the simple amplitude modulation is insufficient. A commercial device for objectively determining the audible threshold based on AMFR is not known to this date.
It is an object of the invention to develop an objective frequency-specific hearing test procedure using the Amplitude Modulation Following Response (AMFR) that can be performed completely automatically and allows a reliable determination of the frequency-dependent audible threshold.
A solution of the object of the invention is recited in claim 1. Further embodiments of the invention are recited in the dependent claims. With the known simulation using an amplitude-modulated carrier, only a small range is activated on the basilar membrane of the inner ear corresponding to the small spectral width of the stimulus (FTrxc2x1FMod). The amplitude of the derived sum response is therefore small. A greater response amplitude can be expected if the spectral width of the stimulus is increased, while recognizing that the response still remains limited to exactly a single spectral line.
According to the concept of the invention, instead of a single amplitude-modulated carrier, several carriers, preferably 3 or 5 carriers, having a frequency that is offset by a frequency difference xcex94F, preferably xcex94F=FMod or xcex94F=2 FMod, are used as a stimulus, wherein all carriers with the same modulation frequency FMod are modulated with an amplitude swing between 30% and 100%, preferably 100%. In addition, one or several of the amplitude-modulated carriers, which for an odd number of carriers are preferably the carriers located between the outermost carriers, can be frequency-modulated, wherein a frequency swing between 0% and 30%, preferably 20%, is selected. With these preferred parameters, the frequency specificity of the measurement is not substantially limited by the spectral width of the stimulus.
Unlike conventional methods, which decide xe2x80x9cAMFR present/not presentxe2x80x9d in a single statistical test, it is proposed to subject the data to several, preferably four, statistical test methods and to assume that an AMFR is present as soon as at least two of the test indicate significance. Determination of the audible threshold can either be performed completely automatic or can be controlled by the examiner.
If the audible threshold is determined by an examiner, then it is proposed to link the PC interfaces which a separate in conventional solutions, for selecting the stimulus (frequency, stimulus level) and displaying the result (audiogram) in the following fashion: a respective audiogram form (DIN standard) is displayed on the PC monitor for the right and left ear, or alternatively can be switched between the right and left ear. An acoustic stimulus with a specified frequency and a specified level is selected by clicking with the mouse on the intersection of the vertical scan line for the frequency marked on the abscissa with the horizontal scan line for the stimulus marked on the ordinate. Alternatively, the intersection can be selected by the arrow keys on the PC keyboard. When using several test frequencies simultaneously for the hearing test (Lins and Picton 1995), a second and, optionally, a third stimulus parameter pair is preselected in the same way. After the start of the measurement and the statistical test, the result (a response to the stimulus is present/not present) is marked on the audiogram form at the intersection of the test frequency and the stimulus level so that the presence or absence of a response to the selected stimulus parameters can be detected.
Applying the solution of the invention has following advantages:
The width of the stimulus spectrum can be preset exactly
The spectrum is almost a rectangle, i.e., all the spectral lines forming to spectrum have almost the same amplitude
The response is limited to only the modulation frequency
On average, the SNR characterizing the response amplitude is greater by a factor 2 than for a conventional simulation using a single amplitude-modulated carrier and also greater than the response obtained with an additional frequency modulation of the single amplitude-modulated carrier. The scatter in the difference between the subjectively and objectively determined audible threshold occurring in the conventional stimulation is thus significantly reduced, so that the objective hearing test method can be used in practice
By using known statistical test methods with AMFR detection, a fully automatic objective frequency-specific hearing test can be implemented
The proposed solution has a lower probability for a fault-positive test result than a process where the decision xe2x80x9cAMFR present/not presentxe2x80x9d is based on a single test, or where four difference statistical test methods (e.g.) are applied and the presence of an AMFR is postulated as soon as two of the tests indicate a significance
When applying the stimulus configuration of the invention, the simultaneous hearing test proposed by Lins and Picton (1995) using several different test frequencies can also be implemented without limitation
A different spectral width of the stimulus can be set that is tuned to the frequency range to be tested
By linking the selection of the stimulus parameters with the display of the results, there is no longer a need to switch between displaying the result (determining which stimulus parameter is to be selected next) and the menu for selecting the stimulus parameters. This prevents arrow""s in the selection of the stimulus parameters, a particular for a simultaneous hearing test with several frequencies
With the proposed linkage of selection of stimulus parameters with the display of the result, both the stimulus parameter frequency and the stimulus level are selected with a single mouse click. The graphic display of the result indicates immediately which stimulus parameters should be selected for the next step in the examination.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.